1. Field of the Invention
This invention relates to a semiconductor device used in power control.
2. Background Art
The ON resistance of a vertical power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) greatly depends on the electrical resistance of the conduction layer (drift layer). The impurity concentration that determines the electrical resistance of the drift layer cannot be increased above a limit according to the breakdown voltage of the p-n junction formed by the base layer and the drift layer. Therefore, a tradeoff relationship exists between the device breakdown voltage and the ON resistance. It is important to improve this tradeoff in devices of low power consumption. The tradeoff includes a limit determined by the device material. Overcoming this limit leads to the realization of a low ON resistance device superior to existing power devices.
To solve these problems, a known example of a MOSFET includes a p-type pillar layer and an n-type pillar layer buried in the drift layer in a structure called a super junction structure. The super junction structure has the same amount of charge (impurity amount) in the p-type pillar layer and the n-type pillar layer and thereby creates a pseudo-non-doped layer, holds a high breakdown voltage, and passes a current through the highly doped n-type pillar layer to realize a low ON resistance superior to that of the material limit. Thus, the tradeoff between the ON resistance and the breakdown voltage superior to those of the material limit is possible to be achieved based on the super junction structure.
When the power MOSFET is used for a bridge circuit and synchronous rectification or the like, a built-in diode may be operated. Usually, bipolar operation is necessary for large current operation of the diode with low ON voltage. However, if the ON resistance is reduced by using the MOSFET having the super junction structure, the low ON voltage can be achieved in spite of unipolar operation of the built-in diode.
For example, JP-A 2007-299970 (Kokai) discloses a structure of a MOSFET having a built-in Schottky barrier diode (a structure having a MOSFET and a Schottky barrier diode mixed loaded on one chip). However, a region serving as the MOSFET and a region serving as the Schottky barrier diode are formed separately in one chip, causing concern about reducing an effective area of the MOSFET to increase the ON resistance.